Multiple explosions were heard around Kyiv on Thursday as invading Russian forces continued marching toward the Ukrainian capital.
An adviser to the presidential office said he feared that Russian forces could be airdropped into the country and then try to penetrate the government district in Kyiv.
Russia launched an all-out invasion of Ukraine by land, air and sea, the biggest attack by one state against another in Europe since World War Two and confirmation of the worst fears of the West.
Ukraine fought Russian forces along practically its entire border with Russia, and there was fierce fighting in the regions of Sumy, Kharkiv, Kherson, Odessa and at a military airport near Kyiv
Ukrainian President Volodymyr Zelensky said that Kyiv broke off diplomatic ties with Moscow amid the fighting and called on all citizens who were ready to defend the country from Russian forces to come forward, saying Kyiv would issue weapons to everyone who wants them.
The Ukrainian government announced around Thursday noon that over 40 Ukrainian service members have been killed by Russian fire and dozens of others were wounded.
A senior Ukrainian government official reported there were also civilian casualties. One of them is a boy who was killed in eastern Ukraine's Kharkiv region after shelling struck an apartment building.
Black smoke was seen rising over the Ukrainian defense ministry intelligence headquarters in central Kyiv on Thursday, although the building appeared to be intact, a Reuters correspondent reported.
Ukraine said on Thursday some of its military command centers have been hit by Russian missile strikes as Moscow launched a military operation against its neighbor.
However, the military intelligence HQ building was not damaged and the smoke appeared to rise from a large fire next to the building into which uniformed people were throwing bags.
Regional authorities of Ukraine's southern Odessa region said that 18 people were killed in a missile attack.
At least six people were killed in Ukraine's town of Brovary, located near the capital, authorities from the town said.
Meanwhile, Russian news agency TSS reported that two Russian ships had been hit by Ukrainian missiles in the Sea of Azov north of the Black Sea, causing several casualties. The report claimed the vessels that were hit were civilian commercial ships.
Russian President Vladimir Putin authorized "a special military operation" against Ukraine on Thursday morning to eliminate what he called a serious threat, saying his aim was to demilitarize Russia's southern neighbor.
In an early-morning address on state television, Putin said he had been left with no choice but to launch the operation, the scope of which was not immediately clear but appeared to go well beyond helping Russian-backed separatists in eastern Ukraine.
"I have decided to conduct a special military operation," said Putin, seated at a desk in the Kremlin next to a battery of telephones, with the Russian flag behind him.
"Its goal is to protect people who have been subjected to bullying and genocide... for the last eight years. And for this, we will strive for the demilitarization and denazification of Ukraine.
"And to bring to court those who committed numerous bloody crimes against civilians, including against citizens of the Russian Federation."
Later Thursday, the speaker of Russia's lower house of parliament, Vyacheslav Volodin, said reiterated Putin's stance that the only way to end hostilities was the full demilitarization of Ukraine.
The Kremlin said that the length of the military operation in Ukraine depended on how it progressed and on its aims, and that the assault should ideally cleanse the country of "Nazis" and "neutralize" Kyiv's military potential.
In a conference call with reporters, Kremlin spokesperson Dmitry Peskov said that nobody was talking about occupying Ukraine and that it was "unacceptable" to use the word to describe in the context of Russia's operation.
British Prime Minister Boris Johnson called the invasion "a catastrophe for Europe" and called for an urgent meeting of NATO leaders and setting out plans to make a public address and speak to G7 leaders.
"This is a catastrophe for our continent," he said on Twitter. "I will make an address to the nation this morning on the Russian invasion of Ukraine."
"I will also speak to fellow G7 leaders and I am calling for an urgent meeting of all NATO leaders as soon as possible."
Meanwhile, European Union leaders are slated to discuss new, tough sanctions on Russia at an emergency meeting later on Thursday in reaction to its "barbaric attack" on Ukraine.
"President Putin is responsible for bringing war back to Europe," European Commission chief, Ursula von der Leyen said, adding that the EU would hold him "accountable for that."